Multiple decoupled antennae connected to radio by different length feeder for impedance mismatch reduction



March 3, 1959 GUANELLA 2,876,448

MULTIPLE DECOUPLED ANTENNAE CONNECTED TO RADIO BY DIFFERENT LENGTHFEEDER FOR IMPEDANCE MIAMATCH REDUCTION Filed March 30, 1954 ATTORNEYUnited States Patent C MULTIPLE DECOUPLED ANTENNAE CON- NECTED TO RADIOBY DIFFERENT LENGTH FEEDER FOR IMPEDANCE MISMATCH RE- DUCTION GustavGuanella, Zurich, S'wwitzerland, assignor to Radio Patents Company, NewYork, N. Y., a partnership Application March 30, 1954, Serial No. 419,792 Claims. (Cl. 343-853) The present invention relates to multipleantenna systems for both directional and omni-directional transmissionand/or reception of radio signals, more particularly to antenna systemsdesigned for operation in the VHF and microwave range of operatingfrequencies.

As is well known, great difficulties are experienced in practice inmatching the impedance of an antenna with its feeder or transmissionlines connecting it to a high frequency transmitter or receiver. In caseof an impedance mismatch between the antenna and its feeder, reflectionsoccur which in turn cause losses in the line and often are the result ofdistortion of the signals being transmitted or received. Furthermore, incase of a transmitting antenna connected to a microwave generator, thereflections have the further effect of unfavorably reacting upon thegenerator (so-called long line effect). While this reaction can beminimized by a sufiiciently loose coupling between the feeder input andthe transmitter, this results in a reduction of the power radiated bythe antenna.

An important object of the present invention is, therefore, theprovision of simple and efficient means in connection with a multipleantenna structure, in particular for use in the VHF and microwave range,whereby the effect of undesirable reflections due to impedance mismatchis practically eliminated or compensated.

The invention, both as to its further objects and novel aspects will bebetter understood by the following detailed description taken inconjunction with the accompanying drawing the several views of whichdiagrammatically show various practical embodiments of multiple antennaeconstructed in accordance with the principles of the invention.

As is well known, the input impedance Z of a loaded transmission linecan be determined to a suflicient approximation by the followingequation:

wherein Z represents the wave impedance, L the electrical length, 8+jathe transmission constant of the line and R is a magnituderepresentative of the mismatch between the load and the line.

For very short waves, such as microwaves, the transmission constant maybe represented simply by 'y=]'a with suflicient approximation, wherebyIn accordance with the present invention, the input impedance of amultiple antenna system designed for an operating wavelength A isrendered independent of the magnitude R representing the mismatch bystaggering or varying the electrical length of the feeders for thevarious antennae by consecutive increments equal to H211, wherein nrepresents the number of antennae of the system. The input ends of thefeeders are preferably connected in series, but if desirable may beconnected in parallel.

In the following the function of the invention is further explained withreference to a simple practical example comprising two antennae withfeeders such as a pair of dipoles arranged for directional,omnidirectional or any other kind of transmission or reception. It isassumed that the first antenna is connected to a feeder having anelectrical length L. Accordingly its input impedance Z is given by theabove Equation 1. The second antenna is provided according to theinvention with a feeder having an electrical length Accordingly, theinput impedance of this feeder is as follows:

By connecting both lines or impedanccs'l and Z in series, the resultanttotal input impedance of the antenna system will be as follows:

From the above it is seen that the input impedance of the system as seenfrom the transmitter or receiver is free from the effect of themagnitude R representing the mismatch or reflection, the systemfunctioning by causing a wave reflected from any of the antennae toconsecutively pass through all the feeders, in such a manner antennaemay be positioned in proper relative space arrangement to obtain adirective transmission or reception in a preferred axis by causing thewaves radiated by the individual antenna to coincide in phase at adesired receiving point, in a manner well known to those skilled in theart. In a simple arrangement the individual antennae of a systemaccording to the invention may be displaced one from the other in thepreferred or directional axis by distancesequal to 7t/2n.

The invention is especially suited for use in connection with antennaarrangements for transmission or re ception of circularly polarizedwaves. For this purpose two antennae, such as two dipoles, are arrangedwith their planes of polarization crossing or forming a right angle.With the feeders of the antenna being designed in the manner accordingto the invention, the effect of misma h or reflection is practicallyeliminated.

The invention will be better understood from the following detaileddescription taken in reference to the accompanying drawing, forming partof this specification and in which:

Fig. l is a general diagram of a multiple antenna system embodying theinvention;

Fig. 2 is a similar diagram for a directional antenna array; and

Fig. 3 is a diagram of a directional antenna system embodying theinvention, comprising a reflector and a plurality of antenna elements.

Like reference characters identify like parts in the different views ofthe drawing.

Referring to Fig. 1 there is shown an arrangement consisting of threesubstantially decoupled antennae, A A and A provided with parabolicreflectors and oriented in different directions. The length s s and s ofthe connected in series,

feeders for the individual antennae to the common junction R is designedto differ from one another by fractional amounts equal to k)\/ 6,wherein it represents the ratio between the velocity of propagation ofthe electric energy and the velocity of light. Since this ratio in thecase of electric waves is equal to l, the difference in the electricallength between the feeders in the example shown is M6. In other words,the reflected oscillations caused by a mismatch of the impedance of oneantenna with its feeder and arriving at the common junction. R, aredisplaced in phase relative to one another by amounts equal to 21r/ 3,in such a manner as to cancel each other in their effect upon the commontransmitter T. The same applies in the case of a plurality of receivingantennae connected to a common receiver. More particularly, for thispurpose the length of the line s in Fig. 1 is equal to s +k)\/6 and thelength of the line .9 is equal to s -l-kM 3 in accordance with theinvention. The input ends of the lines s s and s; which may be in theform of conventional two-wire (Lecher) transmission lines or coaxialcables, are preferably connected in series with each other and thetransmitter T, as pointed out above.

Fig. 2 shows a similar arrangement comprising three conical or V-shapedantenna A A and A arranged to transmit or receive in the same directionand with the electrical lengths of the feeders s s and s again differingone from the other by amounts equal to M6. At the same time the antennaeare shown to be staggered in space by distances equal to M 6, to obtaina directional axis in linewith displacement of the antennae, in a mannerpointed out hereinbefore.

Fig. 3 shows an'embodimeut of an antenna system comprising four helicalantennae A A A and A arranged in spaced relation with their axes atright angle to a reflecting plate or screen H. The energization of theseantennae is effected through four feeders s s s and .9 respectively,whose lengths differ one from the other by amounts equal to M8. Anarrangement of this type has a directional axis which deviates somewhatfrom a line perpendicular to the reflector H. Again, the effect ofmismatch of any of the antennae with its respective feeder will besubstantially compensated or its effect upon the transmitter canceliedor neutralized in substantially the same manner as describedhereinabove.

The individual antennae may be in the form of parabolic radiators,single dipoles or dipole groups and the like, as used in conventionalantenna systems for both directional and omnidirectional transmissionand reception. In other words, the invention applies to any type ofmultiple antenna structure comprising more than a single antenna elementeach connected to a common transmitter or receiver by an individualtransmission line or feeder. By the proper staggering of the electricallengths of the feeders in the manner described by the in vention, theeffect of any impedance mismatch between the antennae with theirrespective feeders is automatically compensated by a neutralization orcancellation of the reflected oscillations at the common input orjunction.

In the foregoing the invention has been described with specificreference to a few illustrative embodiments thereof. It will beapparent, however, that variations and modifications as well as thesubstitution of equivalent elements for those shown, may be made withoutdeparting from the broader spirit of the invention as defined in theappended claims. The specification and drawing are accordingly to beregarded in an illustrative rather than in a limiting sense.

What is claimed is:

1. A multiple antenna system comprising a plurality of more than twoantenna elements and feeders connecting each of said antenna elementswith a common junction point, the improved feature consisting in theelectric lengths of said feeders being consecutively staggered byamounts equal to A/Zn from one to the next antenna element, wherein Arepresents the operating wavelength and n the number of antenna elementsof said system.

2. A multiple antenna system comprising a plurality of more than twoantenna elements being mutually decoupled from one another andindividual feeders connecting each of said antenna elements with acommon operating circuit, the improved feature consisting in theelectric lengths of said feeders being consecutively staggered byrelative amounts equal to M211 between each and the next antennaelement, wherein X represents the operating wavelength and n the numberof antenna elements I of said system.

References Cited in the file of this patent UNITED STATES PATENTS

